If the mind and body were unified, then the manipulations caused by sense faculties could not be overturned by reason. Descartes ascribes the indubitably established in the Cogito to the mind. His consciousness, ability to reason, doubt, will etc must depend on an immutable substance to ensure its existence, like the essence which still made wax after it's external attributes had morphed. Therefore, the body, which he defines as an extension of shape and size and consequent to its nature is divisible, cannot inhere the essence of the "I". .
When understood this way, it gives a more compelling argument to people who want to believe in the soul. The true self is not inhered to a finite concatenation of arms and limbs but within a transcendent spirit. The body is explainable as a divinely created being whose ability to sustain life (eating breathing etc) resides its biological autonomy which does not necessitate will to function. The mind is the imminent faculty of truth which can exist within or without externalities. When my body dies my spirit ascends to heaven.
However, these abstractions were only claims which cannot be proven against his own principles of causation. According to his theory of causality, X occurs because of an intelligible connection with the effects of Y. However, the mind and the body lack this connection because they are comprised of different substances. Descartes attempts to resolve the disparity connecting the mind and body to an area of the brain( the "common sense") which would allow the two to commingle. For example, when my body experiences a sensation such as my arm being pinched, that sensation is transmitted to the brain which in turn orders a reaction; I pull my arm away. This action was evidence of my desire to remove myself from pain that came from my thinking mind and was interpreted through the brain to induce physical interaction between myself and the external stimulas.